This invention relates generally to operational amplifier circuits and, more particularly, to an input stage including junction field-effect-transistors (JFETs) for use in a low-noise, high-frequency operational amplifier.
A typical JFET input stage for an operational amplifier includes first and second differential input JFETs which drive a bipolar current mirror. This arrangement has been satisfactory for operation at reasonable speeds. However, to be suitable for low noise applications, the effective source resistance for each of the JFETs must be relatively low (e.g. 500 ohms). This requires large JFET devices and large currents flowing into the bipolar mirror which produce noise currents which are reflected back into the JFETs and become very significant at low frequencies (e.g. 10 Hz). To reduce this problem, JFET input stages have been designed wherein the bipolar current mirror is replaced by other circuitry including bipolar transistors; however, their emitter-collector paths are not in series with the JFETs and therefore need only conduct small currents resulting in very small noise currents. While this arrangement reduces noise, the JFETs must drive high impedance nodes resulting in high gain from the gate to drain. The gate/drain parasitic capacitance of each JFET is multiplied by this high gain of the JFET forming a pole at the gate which substantially degrades the frequency performance of the circuit.